This invention relates generally to expandable intraluminal vascular grafts, commonly referred to as stents, and more particulary pertains to the coating of stents with materials that allow for the controlled release of pharmacological agents.
Stents are implanted within vessels in an effort to maintain the patency thereof by preventing collapse and/or impeding restenosis. Implantation of a stent is typically accomplished by mounting the stent on the expandable portion of a balloon catheter, maneuvering the catheter through the vasculature so as to position the stent at the treatment site within the body lumen, and inflating the balloon to expand the stent so as to engage the lumen wall. The stent deforms in the expanded configuration allowing the balloon to be deflated and the catheter removed to complete the implantation procedure. The use of self-expanding stents obviates the need for a balloon delivery device. Instead, a constraining sheath that is initially fitted about the stent is simply retracted once the stent is in position adjacent the treatment site. Stents and stent delivery catheters are well known in the art.
The success of a stent placement can be assessed by evaluating a number of factors, such as thrombosis, neointimal hyperplasia, smooth muscle cell migration and proliferation following implantation of the stent, injury to the artery wall, overall loss of luminal patency, stent diameter in vivo, thickness of the stent, and leukocyte adhesion to the luminal lining of stented arteries. The chief areas of concern are early subacute thrombosis, and eventual restenosis of the blood vessel due to intimal hyperplasia.
Therapeutic pharmacological agents have been developed to address some of the concerns associated with the placement of a stent and it is often desirable to provide localized pharmacological treatment of a vessel at the site being supported by the stent. It has been found convenient to utilize the implanted stent for such purpose wherein the stent serves both as a support for the lumen wall as a well as delivery vehicle for the pharmacological agent. However, the metallic materials typically employed in the construction of stents in order to satisfy the mechanical strength requirements are not generally capable of carrying and releasing drugs. On the other hand, while various polymers are known that are quite capable of carrying and releasing drugs, they generally do not have the requisite strength characteristics. Moreover, the structural and mechanical capabilities of a polymer may be significantly reduced as such polymer is loaded with a drug. A previously devised solution to such dilemma has therefore been the coating of a stent's metallic structure with a drug carrying polymer material in order to provide a stent capable of both supporting adequate mechanical loads as well as delivering drugs.
Various approaches have previously been employed to join drug-carrying polymers to metallic stents including for example dipping, spraying and conforming processes. Additionally, methods have been disclosed wherein the metallic structure of the stent has been formed or treated so as to create a porous surface that enhances the ability to retain the applied materials. However, such methods have generally failed to provide a quick, easy and inexpensive way of loading drugs onto a stent, have been limited insofar as the maximum amount of drug that can be loaded onto a stent and are limited in terms of their ability to control the rate of release of the drug upon implantation of the stent. Additionally, some of the heretofore known methods are highly specific wherein they are substantially limited in terms of which underlying stent material the coating can be applied to.